A communication system can be full duplex or half duplex. In a full duplex system each communicating transceiver can receive and transmit signals simultaneously, typically in distinct frequency bands (frequency division) or by using different codes (code division) or combinations of these. In contrast, in a half duplex system each transceiver receives and transmits signals in separate time periods, typically within a single frequency band. The former approach has several advantages due to its relaxed need for time synchronization, its higher efficiency due to lack of guard times, the avoidance of slot period related interference, cell planning advantages etc. Hence most current cellular standards have adopted the use of full duplex communication. However, a full duplex receiver is susceptible to a problem called “self-interference”. Typically, full duplex transceivers resolve the self-interference problem by suppressing signals in the transmit frequency band at the receiver input, often requiring advanced, non-integrateable, expensive and bulky devices such as Surface Acoustic Wave (SAW) filters. Clearly, with the continuous drive towards lower cost and smaller footprint, the problem of reducing self-interference is more pressing than ever.
Secondly, MIMO techniques have been introduced due to the radio wireless channel environment existing in a mobile communication system is subject to inevitable errors. These can be due to several factors such as multipath interference, shadowing, propagation attenuation, time-varying noise, interference and fading, all of which result in a deterioration in the overall performance of the mobile communication system.
Generally, in order to reduce the information loss, various channel coding techniques are used to enhance system reliability. To improve performance beyond what is achievable with channel coding and to increase the communication system's spectral efficiency, diversity schemes are used for improving communication robustness even in the presence of fading. Diversity schemes are roughly classified into time, frequency and antenna/space diversity schemes. The antenna diversity scheme, which uses multiple antennas, is in turn classified into a receive antenna diversity scheme using a plurality of receive antennas (MISO), a transmit antenna diversity scheme using a plurality of transmit antennas (SIMO) and a Multiple Input Multiple Output (MIMO) scheme using a plurality of receive and transmit antennas at the transmitter and a receiver respectively. Due to the nature of the radio wireless propagation channel, the radio wireless channel between any pair of transmit-receive antennas is different, which results in so-called spatial diversity. Several factors are influencing the degree to which these antenna pairs are uncorrelated, and the better uncorrelated they are, the more received signal quality gain can be obtained using signal processing exploiting the signals received at the different antennas. As an example, in its most simple form the signal processing may consist of employing an antenna selection method in which some antennas having good channel conditions are selected from among the antennas of the transmitter or the receiver, and signals are transmitted/received through the selected antennas. In more complex schemes, the transmit side may deliberately introduce diversity and/or create known relationships between the signals transmitted on the different antennas. With data rate requirements steadily increasing while coverage expectations remain at similar levels to legacy systems, all of the above techniques are gradually being introduced in radio wireless communications systems, such as those used for cellular communications. Of particular relevance is that the introduction of these techniques comes at a cost and footprint penalty, both very sensitive parameters in the modern communications systems transceivers.
EP1926336 describes MIMO techniques to increase capacity and quality between plurality of users and a basestation. U.S. Pat. No. 5,691,978 relates to duplexing on the same band. It uses active cancellation techniques after the facts (post-cancellation) and does not rely on having multiple transmit antennas.
U.S. Pat. No. 7,155,179 describes a full duplex transceiver having a method for immunizing itself against self-jamming. It uses a centre frequency of jamming signal to down convert desired signal. It works for single antenna systems and requires significant dynamic range up to the high pass filter.